Calcium(Ca) is a secondary nutrient and one of the immobile elements in a marijuana plant. It is used in making the plants stems and branches much stronger (think strong bones in people) as well as giving a boost to the plants root growth helping them grow the newer root hairs. Calcium(Ca) tends to hang around the roots and older growth. A shortage of this nutrient can make stems that can't hold the plant up.
The PH starts locking out Calcium(Ca) from soil at 6.4 and below and 7.7 and above. Hydro and Soil less mediums experience lockout at below 4.6 and higher than 5.8.
A deficiency can be fixed by foliar feeding (adding liquid fertilizer directly to plant leaves) one teaspoon of dolomatic lime per quart of water until the plant's condition improves.
Well now our beautiful Central Coast as you know by now has numerous grow shops specializing in our choice of gardens. The experts that work there have a lot of experience doing exactly what you are attempting to do right now. Let's listen in on some of these experts in their full time field.
Starting with Nick from Garden All Year he says that he loves azomite which is actually an acronym for the "A to Z of minerals including trace elements. It comes from mines up in Utah where a volcano erupted about 30 million years ago and filled a seabed with volcanic ash. The nutrient rich water with the ash combined over the years and created a wonderful addition to any growers portfolio. It is said to contain over 70 trace minerals as well as many rare earth elements. Nick also being a firm believer in compost teas he likes using worm castings and General Hydroponics forest humus to bring up the trace elements as well. Also Dyna-Gro makes their Foliage-Pro which contains all the needed micro nutrients that your plants will love.
Down in SLO town at Healthy Harvest Grady says he uses an all around micro nutrient providing product from Botanicare called CNS17. It can be used in your water to get right to the roots or foliar fed to your plants in a spray which will give a quicker reaction by your plants.